Project Summary/Abstract: While the proper selection of splice sites drives genomic diversity, adaptive growth and development, errors in splicing can have enormous detrimental effects on function and is now recognized as the underlying cause for many human diseases. Indeed, splicing errors are associated with muscular dystrophy, Alzheimer's disease, Parkinsonism, psychiatric disorders, ataxias and cancers making the study of factors that control splice-site selection vitally important for human health. Splicing occurs at the spliceosome, a macromolecular complex composed of several RNAs and numerous proteins. Critical to normal gene splicing is the proper selection of the 5'-3' splice sites, events that occur early in the development of the spliceosome and whose specificity is guided by an essential family of splicing factors known as SR proteins. The phosphorylation states of SR proteins directly impact their subcellular localization and splicing activities but our understanding of how these different forms are attained is, at best, incomplete. The CLK family of protein kinases phosphorylates SR proteins and mobilizes them to sites of active gene splicing. The CLKs differ from many classic protein kinases in that they lack a docking groove for substrate binding but instead contain a disordered N-terminal extension that we showed attaches to the SR protein. In this proposal we will explore the role of the N-terminus for the mobilization of CLK1 and recognition of SR proteins in the nucleus using a wide array of in vivo and in vitro experiments. We will study the effects of CLK-dependent phosphorylation on SR protein conformation, subcellular localization, and interactions with critical mediators of splice-site selection in the spliceosome. The larger goal of this proposal is to define CLK function at both biological and biochemical levels so that we can better understand the mechanisms of human diseases associated with errors in splicing.